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This invention relates to image display systems and, in particular, to an image projection system implemented with a reflective light modulator and having an optical system that reduces weight and packaging profile.
The following description is presented with reference to an image projector implemented with a reflective light modulator of a digital micromirror device (xe2x80x9cDMDxe2x80x9d) type but is applicable also to image projectors implemented with other types of reflective light modulators. Image projectors currently implemented with DMDs require that the projector housing or DMD-illuminating light beam-directing optics contained within the projector housing be tilted at a 45 degree angle relative to a support table on which the image projector rests. This is done to cause the illuminating light to impinge on the DMD from either above or below its light reflecting surface and thereby provide a correct orientation of the DMD relative to a projection screen on which an image can be viewed. Inclining the projector or its components causes the projector to occupy an undesirably tall space when it is in use. Currently available single DMD projectors are taller than 10 cm in their operating positions. Using a tilting mechanism to thin the profile to less than 10 cm requires a tilting mechanism that raises the operating height by a corresponding amount.
FIGS. 1A, 1B, 1C, and 1D are respective isometric, frontal, side elevation, and top plan views of such a prior art image projector 10, which includes a high power lamp 12 positioned at the focus of an elliptical reflector 14 to produce a high intensity illumination beam characterized by a principal ray 16 that propagates through a rotating color wheel disk 18 of a color wheel assembly 20. Disk 18 includes at least three sectors, each tinted in a different one of three primary colors to provide a field sequential color image capability for image projector 10. The illumination beam propagates through an integrator tunnel 22 to create at its output end a uniform illumination pattern that lens elements 24, 26, and 28 image onto a DMD 30.
The illumination beam propagating from integrator tunnel 22 is directed by a mirror 32 that is inclined so that the illumination beam propagates upwardly at a 45 degree angle relative to the plane of the supporting table for image projector 10 and exits lens element 26 toward a prism assembly 40. Prism assembly 40 is composed of prism components 42 and 44 that are spaced apart by an air space interface 46. After reflection by mirror 32, principal ray 16 of the illumination beam strikes a surface of lens element 28.
An incident light beam derived from principal ray 16 propagates through prism component 42 and, by total internal reflection, reflects off of a surface 50 at air space interface 46 to form a reflected incident light beam. The reflected incident beam propagates through prism component 42 to strike DMD 30. DMD 30 in its xe2x80x9conxe2x80x9d light reflecting state (on-state) reflects an imaging light beam propagating normal to the plane of DMD 30 through prism component 42 and, without total internal reflection, through air space interface 46 into prism 44 to exit through an exit face 60 of prism component 44. The imaging light beam that passes through exit face 60 is characterized by a principal ray 62 and propagates through a projection lens 64 to a projector screen (not shown) to display an image to a viewer. DMD 30 in its xe2x80x9coffxe2x80x9d light reflecting state (off-state) reflects light by total internal reflection off of a face 68 of prism component 44.
The angles of the faces and the shapes of prism components 42 and 44 are selected so that the incident light beam, reflected incident light beam, and imaging light beam propagating within prism assembly 40 are coplanar. The arrangement of the components of image projector 10 results in the upward inclination of prism assembly 40 and thereby dictates for a housing (not shown) of projector 10 a minimum height that is greater than a minimum height that would be possible with an uninclined prism assembly and principal rays 16 and 62 propagating along essentially the same vector.
Reducing its height is one step in achieving true portability for image projector 10. Reducing its weight (mass) would be another beneficial step toward true portability. The image projector market is demanding projectors that can be carried along with a companion laptop computer in a briefcase.
An object of this invention is, therefore, to provide a truly portable image projector apparatus.
Another object of this invention is to provide an image projector weighing less than about 2.27 kilograms (five pounds) and having a height less than about 6.35 centimeters (2.5 inches).
A further object of this invention is to provide an image projection optical system suitable for use with a light-weight and compact projection lens.
This invention is suitable for use in an image projector employing a reflective light modulator, such as a DMD, and a prism assembly that illuminates the DMD and receives a reflected imaging light bundle for directing toward a projection lens.
The prism assembly includes compensating and output prism components having opposed surfaces separated by an air gap. The prism assembly sets up a correct illumination angle on the DMD and then separates incident illumination light from reflected imaging light by total internal reflection discrimination. The illumination light enters the prism assembly and reflects by total internal reflection off a top surface of the compensating prism component. The top surface has a compound angle that directs the light toward the DMD at the correct angle for illumination. For each micromechanical mirror of the DMD in its on-state, the illumination light reflects nearly normal to the horizontal datum plane to form imaging light that reenters the prism assembly through the output prism component. Because the angle of incidence at the air gap is greater than the critical angle, the imaging light reflects off the air gap and propagates through the output prism component.
A focusing lens of this invention is disposed between the prism assembly and the DMD to refract all the reflected imaging light into a converging imaging light bundle as it propagates into a small diameter light entry pupil of a light-weight, compact projection lens.
In an alternative embodiment, the focusing lens is implemented as a curved surface on the side of the prism assembly facing the reflective light modulator. This embodiment reduces weight, complexity, and space, but adds to the cost and complexity of the prism assembly.
An advantage of the focusing lens of this invention is that the projection lens requires a light entry pupil having a diameter that is only 30 to 50 percent the diameter of prior art entry pupils.
Another advantage of the focusing lens of this invention is that the projection lens has a mass and a length that is about 50 to 75 percent of prior art projection lenses.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings.